There are several different general types of fuel cells which can be distinguished one from another primarily by the electrolytes they utilize. For example, there are fuel cells which utilize phosphoric acid, sulfuric acid, fluorinated phosphoric or sulfuric acid or the like acid electrolytes; ion exchange electrolytes; molten carbonate salt electrolytes; solid electrolytes such as doped zirconia or ceria; alkaline electrolytes; and other electrolytes which can be used in the electrochemical electricity-producing reaction. All of these types of fuel cells will use hydrogen as the anode reactant and oxygen as the cathode reactant. The hydrogen will typically come from a fossil fuel which has been catalytically converted to a hydrogen-rich fuel gas, and the oxygen will come from air passed over the cathode side of the cell or cell stack. The converted fuel gas, as noted, will be rich in hydrogen, but will also contain a significant percentage of carbon dioxide. Of the cell stack types identified above, none will be adversely affected by the carbon dioxide component of the fuel gas except for the alkaline stacks. In the alkaline stacks, steps must be taken to remove the carbon dioxide from the converted fuel gas before it passes through the anode side of the stack. Thus, the anode exhaust from an alkaline stack will not contain any appreciable amounts of carbon dioxide. Since the remaining types of cell stacks can tolerate carbon dioxide, there has been no motivation in the prior art to remove carbon dioxide from the anode gases in these nonalkaline fuel cell stack systems.
This invention is directed toward the removal of carbon dioxide from nonalkaline fuel cell stack anode gases. There are two basic reasons why carbon dioxide should be removed from nonalkaline fuel cell stack anode gases. Firstly, such removal will lessen the amount of carbon dioxide emitted into the atmosphere by fuel cell power plants. It is well documented that increases in atmospheric carbon dioxide will result in corresponding increases in mean ambient temperatures, due to the so-called "Greenhouse Effect". It is also common knowledge that conventional electric power generating plants emit significant amounts of carbon dioxide into the atmosphere through their flue stacks. Systems which can be used to scrub the conventional power plant effluents of carbon dioxide are expensive and not particularly efficient due to the low carbon dioxide concentrations. By comparison, fuel cell power plants can be inexpensively and efficiently adapted to provide for carbon dioxide removal from their anode gases. The second reason why carbon dioxide removal in nonalkaline fuel cell power plants is desirable, even though carbon dioxide does not adversely affect operation of such plants, is that the overall operating economies of such modified plants are improved. Carbon dioxide is a useful, saleable product, and it is relatively easy to recover in the fuel cell stack environment. As previously noted, the converted fuel gas will be rich in hydrogen and will also contain a significant amount of carbon dioxide as it enters the stack to pass through the anode side. In passing through the anode side of the stack, the percentage of hydrogen in the fuel gas is markedly reduced as it is used in the electrochemical reaction. Thus, the gas exhausted from the anode side of the stack is still a mixture of hydrogen and carbon dioxide, but the percentage of carbon dioxide is greatly increased, and in fact carbon dioxide is a major constituent and can be present in percentages of up to 50%. The carbon dioxide component of this anode exhaust gas can be easily recovered in the fuel cell stack system by a regenerable absorbant. The heat and internal pressure present in the fuel cell stack environment are particularly conducive to the efficient absorption of carbon dioxide in an absorbant, and the subsequent separation of the carbon dioxide from the absorbant in a regenerator. The carbon dioxide retrieved from the regenerator is then cooled and compressed for further utilization in other applications.
It is therefore an object of this invention to provide an improved nonalkaline fuel cell power plant with lower carbon dioxide emissions.
It is a further object of this invention to provide a power plant of the character described which produces high purity carbon dioxide as a byproduct.
It is another object of this invention to provide a power plant of the character described wherein carbon dioxide in the fuel gases is effectively removed from the plant anode exhaust gases.
It is an additional object of this invention to provide a power plant of the character described wherein carbon dioxide is recovered with an absorbant material which is regenerated within the power plant system.
These and other objects and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention when taken in conjunction with the accompanying drawings.